Stretch film is widely used in the wrapping and securing of pallet loads for shipping. It is known that such plastic films, which are commonly made from low density polyethylene (LLDPE), gain significantly in ultimate tensile strength when stretched above their yield point. This is due to the fact that such plastic films have a “memory”. For example, film that is stretched to such a degree so as to create a 10% memory will shrink 10% of its stretched length. This “memory” assists in the holding of pallet loads wrapped with stretch film, as the “memory” or the tendency to shrink assists in securing the palletized articles together under compression.
Stretch film is commonly fabricated by stretching the web of plastic film prior to rolling. This pre-stretching facilitates the process of manually applying of the product to a pallet load, as the end user is required to apply less manual tensile force during the wrapping process. Such pre-stretched rolled stretch film webs are fabricated in the form of rolls 20, as illustrated in FIG. 1, and provide a compact product that may be either applied to a pallet load manually or loaded into an automated wrapping machine.
A number of methods of fabricating pre-stretched stretch film have previously been disclosed. For example, U.S. Pat. No. 5,967,437 to Martin-Cocher, et al. discloses a method of manufacturing rolls of pre-stretched film that comprises importing oscillation to the film with a component perpendicular to the film axis. For example, during pre-stretching and winding of the film on a take-up roll, a feed roll, i.e. a spool from which the stretchable film for stretching is taken, is caused to oscillate, and/or the take-up core is caused to oscillate. This ensures that successive layers of film are offset, in particular at the margins of the film. Rolls of film obtained in this way do not have portions of extra thickness and lower tension at the margins.
U.S. Pat. No. 6,592,699 to Mehta, et al. discloses a highly stretched film that has permanently heat-sealed edges which are nearly invisible. A folding apparatus folds the edge portions of a film to form folded edges. The folded film then is passed through a bridle mechanism having two heated rolls. A series of nips apply pressure to the folded film as it is heated by the heated rolls. The nip pressure, combined with the heat imparted by the heated rolls, causes thermal welding of the folded edges resulting in permanent heat-sealed edges. Because the film edges are folded prior to the hot-stretching, the hot stretch process performs dual functions of stretching the film and heat sealing the folded edges. The resulting stretched film has permanently heat-sealed edges that do no separate, and which prevent tearing of the film edges during subsequent processing and use.
Stretch film has a tendency to retain a “memory” after rolling, which tends to cause the stretch film to contract into its original length as the film recovers, and which in turn can cause adjacent layers of rolled, pre-stretched film tend to fuse together. This can make it difficult to unroll pre-stretched film without tearing the film. Additionally, rolls of stretch film made by conventional fabrication methods are tightly wound, and are dense, solid items. Such stretch film rolls are typically not tolerant to mishandling such as accidental dropping, and can suffer damage such as denting. Denting can cause the film to tear during use (i.e. during the wrapping of a pallet load) which effectively terminates the usage lifetime of the roll and results in waste.
It is an object of this invention to provide an improved roll of stretch film that has a high tolerance to mishandling, and a method of fabricating the same.